Monoclonal antibodies prepared to spermatozoa, spermatogenic cells, or isolated antigens are being used to identify and characterize molecules specific to germ cells. The general hypothesis being tested is that germ cell-specific gene products are responsible for the unique structural and functional characteristics of spermatozoa. The fibrous sheath is a cytoskeletal structure which underlies the plasma membrane and surrounds the outer dense fibers and axoneme in the principal piece of the mammalian sperm flagellum. A monoclonal antibody was found to recognize a protein of apparent Mr 66.7 K and pI 8.5 using 2D SDS-PAGE and Western blotting procedures. Antigen appearance during spermatogenesis was analyzed using germ cells isolated from juvenile and adult mice. The 66.7 K antigen first appears in found spermatids. However, a 78 K antigen recognized by the same antibody and present throughout spermatocyte development disappears between the round spermatid and elongating spermatid stages. Use of other antibodies indicates that this is due to epitope modification rather than degradation and that this protein also becomes incorporated into the fibrous sheath. The hypothesis currently being tested is that the fibrous sheath protein is a germ cell-specific intermediate filament protein. Studies on sperm surface components involve a 31 K glycoprotein that appears during epididymal maturation. A monoclonal antibody has been used to show that the antigen appears in the corpus epididymidis on the plasma membrane of the flagellum. This antigen is shed from sperm under in vitro fertilization conditions but retained in the presence of epididymal fluid. The antigen stabilizing factor is being purified from epididymal fluid and from Sertoli cell conditioned medium to study the nature of the molecule and its role in epididymal maturation and capacitation.